Add non-root pantsbuild user to remote execution image (#10583)
Add non-root pantsbuild user to remote execution image (#10583)

### Problem

The remote execution image used on RBE ran as root. For various reasons (including security), we should run programs in containers using this image as a non-root user.

### Solution

Add a non-root `pantsbuild` user and make the image use it.

### Result

Pants does not run as root in remote execution.

benjyw/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,jsirois/pants,pantsbuild/pants,jsirois/pants,jsirois/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants